Equipment—Etching

The Plasma Lab 80 Plus uses a plasma to create ions from gas molecules such as Oxygen (O2), Tetrafluoromethane (CF4), Trifluoromethane (CHF3) and Sulfur Hexafluoride (SF6). The ions are driven by the eclectic field toward the sample surface where they collide and cause some of the sample atoms to be blasted away (just like sputtering.) The ions also react chemically with the surface where the products, which include some of the surface material, are all volatile and result in etching of the surface. If the surface is coated with a mask, a patterned polymer layer, the system can be set up so that the exposed surface etches much more quickly than the mask; when the mask layer is removed the pattern is left etched into the sample surface.

The Plasma Lab ICP 180 is also a reactive ion etcher but it has the capability to do Bosch etching and cryo etching. Ideally any etching method would etch straight down to create a trench with steep sidewalls; in reality the sides of the trench are also etched. Reactive ion etching can make trenches with steep side walls but Bosch and cryo etching can do much better. The Bosch process works by alternating between a reactive ion etch step and a step where octofluorocyclobutane (C4F8) reacts on the surface to create a protective polymer layer. During the etch step most of the ions target the bottom of the trench so the protective layer is destroyed and etching continues but the protective layer on the sidewall holds and prevents etching from occurring. In the cryo process, the sample is chilled to cryogenic temperatures which slows down the chemical reaction between the ions and the surface; etching is done primarily by sputtering which favors the bottom of the trench over the sidewalls. With these two techniques trenches can be made 20 times as deep as they are wide.

A broad beam of neutral Argon atom bombards the surface of a sample, abrasively removing material. This system is used specifically to mill metals which are not effectively etched by RIE. It’s called an ion mill because the argon atoms are initially ionized but are neutralized by electrons before reaching the surface to be milled.